This invention relates to a catalytic process for the methylation of the ortho position of phenols having at least one ortho hydrogen and to certain novel catalysts useful in the process.
Compounds such as those represented by the general formula: ##SPC1##
Are of known utility. They are especially useful in the preparation of polymers such as polyphenylene oxide. In the formula each R is a monovalent substituent selected from the group consisting of hydrogen, methyl, phenyl, and methyl substituted phenyl.
Methods are already available for the alkylation of phenols in the ortho position. They are described, for example, in British Pat. Nos. 717,588 and 1,112,914; U.S. Pat. No. 3,446,856; Japanese Pat. Publications 27367/1964 and 29293/1970; and in the report by Enomoto et al. in the Catalyst Research Conference of 1968.
British Pat. No. 717,588 relates to an alkylation process in which a metal oxide having dehydrating action such as the oxides of aluminum, thorium, zirconium, zinc, iron, manganese, magnesium or calcium are employed as catalysts. The process is not completely satisfactory, however, because it is not sufficiently selective. For example, the methyla- of phenol leads not only to the desired ortho-substituted products but also to undesirably large amounts of the meta and para isomers. The separation of the undesired byproducts is complicated and expensive, but they must be separated especially if the desired product, 2,6-xylenol is to be used for the production of polyphenylene oxide.
U.S. Pat. No. 3,446,856 describes the alkylation reaction using magnesia as a catalyst. This is a somewhat improved procedure, but the elevated temperature, e.g. 475.degree.C. at which it is carried out leads to excessive decomposition of starting materials such as phenol and methanol.
British Patent No. 1,112,914, Enotmoto's method, and the above identified Japanese Patent Publications all relate to improved processes in which the magnesia is enriched with a second or third element. With relatively inexpensive additives there is still appreciable decomposition of the starting materials which is often accompanied by a marked decrease in the activity of the catalyst. The addition of rare earth metals or of uranium or palladium is helpful, but it is also expensive.
Despite extensive research no completely satisfactory method has yet been found for the ortho-methylation of phenols.